a) Field of the Invention
The present invention relates to a slidable-on-a-shaft type objective lens driving system in an optical pick-up applied to an optical disk apparatus and the like. More specifically, it relates to a modulation technique to optimize the resonance frequency of the lens holder which holds the objective lens in this type of objective lens driving system.
b) Description of the Related Art
As shown in FIG. 1 or 7 in U.S. Pat. No. 4,998,802, in the slidable-on-a-shaft type objective lens driving system in an optical pick-up, the lens holder 1 (a movable member) is slidably held along the journal 7 formed on a fixed member and rotatably held around the journal 7. Constructed between the lens holder 1 and the fixed member are magnetic driving circuits for focusing and for tracking. In general, a driving coil for tracking 3 and a driving coil for focusing 4 are mounted on the outer periphery of the lens holder 1; at the positions on the fixed member opposite the aforementioned coils, a magnet for tracking 6b and a magnet for focusing 6a are mounted. When the driving coil for focusing 4 is excited, the lens holder 1 slides along the journal 7 to correct focusing errors of the objective lens 2 mounted in the lens holder 1. In the same manner, when the driving coil for tracking 3 is excited, the lens holder 1 rotates around the journal 7 to correct focusing errors of the objective lens 2.
At the position on the outer periphery of the lens holder 1, opposite the magnet for focusing 6a, a magnetic piece 10 is mounted as shown in FIGS. 11 and 12 of the U.S. Pat. No. 4,998,802. By using the magnetic force generated between the magnetic piece 10 and the magnet for focusing 6a under the condition the driving coils 3 and 4 are not excited, the lens holder 1 is held at a predetermined neutral position which is a home position for positional control.
In such conventional technology, a magnetic spring is normally working on the lens holder 1; therefore, the relationship between the frequency and amplitude of the vibration when the lens holder 1 freely vibrates is as shown by a solid line L1 in FIG. 4 of this invention. Normally, the size of the magnetic piece 10 is adjusted so that the resonance frequency fo of the lens holder 1 is slightly higher than the optical recording disk's rotations per minute.
In such an objective lens driving system, the attraction force F exerted on the magnetic piece 10 from the magnetic field of the magnet 6 is proportional to the product of the following: the constant of proportionality (K); the volume (V) of the magnetic piece 10; the permeability (.mu.) of the magnetic piece 10; the strength (H) of the magnetic field exerted on the magnetic piece 10; and the magnetic declination (dH/dx) of the magnetic field on which the magnetic piece 10 is arranged. That is, the attraction force F is expressed in the following equation: EQU F.varies.k.times.V.times..mu..times.H.times.(dH/dx) (1)
Then, in order to modulate the resonance frequency of the lens holder 1 from (fo) to (fo'), the magnetic piece 10 is replaced with a smaller piece to minimize the attraction force F and thereby the spring constant of the lens holder 1 is minimized. Consequently, as shown by dotted lines L2 in FIG. 4 of this invention, the resonance frequency of the lens holder 1 can be shifted to the fo' side.
If the resonance frequency of the lens holder 1 is modulated by changing the size and shape of the magnetic piece 10, as is done conventionally, there is a problem in that one cannot predict to which level the resonance frequency shifts. This is because, even if the same volume and the same shape are used for the magnetic pieces, the permeability .mu. sometimes varies due to the manunfacturing hysteresis or distortion of the individual magnetic piece. Accordingly, the attraction force F exerted on the replace magnetic piece cannot be predicted accurately, and therefore the resonance frequency after adjusting the lens holder 1 cannot be predicted.